The present invention relates to a communication system that performs digital data transmission using quadrature amplitude modulation (QAM). The QAM signal is received at a receiving unit and demodulated in order to obtain the digital data. The demodulator typically includes a carrier signal that is synchronized with the QAM signal. The carrier signal is typically generated by a voltage controlled oscillator (VCO).
The carrier signal is used to quadrature-demodulate the. signal and generate baseband signals. The baseband signals typically comprise an I-phase signal and a Q-phase signal. The I-phase and Q-phase signals are amplified and converted to digital output signals that may be plotted in a phase plane with predetermined coordinates. For example, in a 64-QAM there are 64 lattice points, 8 points horizontally and 8 points vertically. The amplitude of the I-phase baseband signal is plotted along the horizontal axis and the amplitude of the Q-phase baseband signal is plotted along the vertical axis.
A phase comparator generates an error signal based on the phase error between the baseband signals. The error signal is filtered and applied as a control signal to the VCO. The frequency of the carrier signal generated by the VCO is adjusted in order to rotate the baseband signals into coincidence with the predetermined lattice points in the phase plane. The Park U.S. Pat. No. 5,572,550 dated Nov. 5, 1996 (xe2x80x9cParkxe2x80x9d) discloses a decision directed carrier recovery circuit as described above and the disclosure thereof is incorporated by reference herein.
It is not uncommon for a communications system to experience a sudden break in communications. When this happens, the carrier signal may be thrown out of synchronization with the modulated data carrying communications signal. The baseband signal rotates about the origin of the phase plane, and the VCO begins to oscillate about the frequency midway between the frequency of the modulated signal and the carrier signal. Once synchronization is broken, restoration generally requires some intervention. The carrier signal must be brought back into synchronization quickly to avoid the loss of data.
Previous systems typically used a sweeper to establish synchronization between the signals. The sweeper was activated when an out-of-synchronization condition was detected and caused the frequency of the VCO to slowly xe2x80x9csweepxe2x80x9d over a predetermined capture range until the phases were matched. Once the phases were synchronized the sweeper was stopped and normal operation of the system was resumed. The time required to reestablish synchronization depended on the sweeper speed. A sweeper operating with a cycle of between several hundred milliseconds to several seconds could take up to several seconds to reestablish synchronization.
The Kiyanagi U.S. Pat. No. 5,594,389 dated Jan. 14, 1997 (xe2x80x9cKiyanagixe2x80x9d) discloses an area judging device for providing a signal that indicates whether the baseband signal lies inside a predetermined area in the phase plane, and the disclosure thereof is incorporated by reference herein. Just before the baseband signal rotates out of the designated area a control signal is generated to force the baseband signal in the opposite direction of rotation. Kiyanagi also discloses holding circuits for maintaining the control signal constant until the baseband signal returns to the designated area by such counter-rotation.
The area system is complex and more difficult to implement than the sweeper system and there remains a need for a simple decision directed system that is capable of quickly restoring synchronization between the carrier signal and the QAM signal.
Accordingly, it is an object of the present invention to provide a novel QAM communication system and method with a demodulator that can quickly resynchronize the carrier signal with the data carrying QAM signal.
It is another object of the present invention to provide a novel QAM demodulator and method that includes a decision-directed carrier frequency detector for a voltage controlled oscillator.
It is another object of the present invention to provide a novel QAM demodulator and method that includes a decision-directed carrier frequency detector for a numerically controlled oscillator.
It is still another object of the present invention to provide a novel alternative to a VCO frequency sweeper system and method.
It is yet another object of the present invention to provide a novel alternative to an area judging system and method.
It is a further object of the present invention to provide a novel decision directed carrier frequency detector and method that can be used with existing QAM demodulation systems.
It is still a further object of the present invention to provide a cost effective alternative to a VCO frequency sweeper QAM demodulator system and method.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, when read in conjunction with the appended drawings and the following detailed description of a preferred embodiment.